1. Field of the Invention
The present invention relates to an asynchronous transfer mode (ATM) communication system, and more particularly, to an image transmission method and apparatus capable of effectively transmitting image data without deteriorating the quality of service (QoS) even when an ATM network is congested.
2. Description of the Related Art
Recently, ATM communication systems have been developed to realize a broadband integrated services digital network (B-ISDN).
In the ATM communication system, all multi-media information related to audio, video and data are stored in fixed length cells each formed by a header (5 bytes) and a payload (48 bytes). Since the cells have the same configuration, it is possible to multiplex, demultiplex and switch in the same network at different rates regardless of the kind of multi-media.
In the ATM communication system, if an abnormally large number of cells are supplied to a network, it is impossible to guarantee quality of service (QoS). This state is called a congested state.
In order to suppress the generation of congested states, various ATM traffic service classes, i.e., a constant bit rate (CBR) service, a real-time variable bit rate (rt-VBR) service, a non-real-time variable bit rate (nrt-VBR) service, an unspecified bit rate (UBR) service and an available bit rate (ABR) service are defined in the ATM communication system. Note that the CBR service is of a band guaranteeing type, and the rt-VBR services the nrt-VBR service, the UBR service and the ABR service are of a best effort type.
Also, when a virtual connection (VC) is established between a terminal and the ATM network, traffic parameters such as a peak cell rate (PCR), a substainable cell rate (SCR) and the like as well as a required quality of service QoS are declared for each of the above-mentioned traffic service classes.
In the CBR service, a fixed cell rate is allocated to each connection between a terminal and the ATM network. The CBR service is applied to a real-time moving image/speech service, a line exchange emulation service and the like. Since the cell rate is fixed, the peak cell rate PCR as well as the quality of service QoS is declared but the substainable cell rate SCR is not declared. However, if one image data is transmitted via the ATM network by using only the CBR service, an enormously large band of the ATM network is exhausted, so that effective use is not made of the band of the ATM network.
In the VBR service, a statistically-determined variable cell rate is allocated to each connection between a terminal and the ATM network. For example, the rt-VBR service is applied to a variable rate video and speech. Also, the nrt-VBR service is applied to a non-real-time type data communication service such as a frame relay service of a public network. In the VBR service class, the peak cell rate PCR and the substainable cell rate SCR as well as the quality of service QoS are both declared.
However, if one image data is transmitted via the ATM network by using only the rt-VBR service or the nrt-VBR service, many other traffic parameters such as a maximum burst size (MBS) are needed to be declared for every frequency component, so that it is difficult to uniformly define a burst model.
In the UBR service, a cell rate is determined by a terminal, that is, the control of cell rates by the ATM network is not carried out. In the UBR service, only the peak cell rate PCR is declared, and other traffic parameters as well as the quality of service QoS are not declared.
However, if one image data is transmitted via an ATM network by using only the UBR service, the important low frequency components are treated equally to the non-important high frequency components. As a result, when the ATM network is congested, the cells of the important low frequency components may be scrapped, which would not guarantee the quality of service QoS.
In the ABR service, an allowed cell rate (ACR) is calculated in accordance with congestion information fed back from the ATM network, and also, the allowed cell rate ACR is changed between a minimum cell rate (MCR) and a peas cell rate PCR. That is,
MCRxe2x89xa6ACRxe2x89xa6PCR
Thus, a feedback operation using the congestion information is performed upon the ACR, which effectively makes use of the bandwidth of the ATM network.
However, if one image data is transmitted via the ATM network by using only the ABR service, the feedback control of congestion information would make the control of the ATM network complex.
It is an object of the present invention to provide an image transmission method and apparatus using an ATM network capable of effectively using the band of the ATM network without reducing the quality of service even when the ATM network is congested.
According to the present invention, in an image transmission method using an ATM network, one image data is divided into a plurality of groups each corresponding to one traffic service class of an ATM layer in accordance with frequency components of the image data, and virtual connections (VCs) are established for the traffic service classes. Then, each group of the image data is transmitted by the corresponding virtual connection. Thus, if a band guaranteeing type traffic service such as the CBR service is defined for the low frequency component of the image data requiring a high quality of service and a best effort type traffic service class such as the VBR service, the UBR service or the ABR service is defined for the high frequency component of the image data requiring a low quality of service, effective use is made of the band of the ATM network.